SSC CGL 2025 Tier-I · 12 September 2025, Shift 1 · General Awareness · Question 24 of 100

Text as printed on a candidate's response sheet for this shift; answer per SSC's tentative key as marked on that sheet. Solution last reviewed 11 September 2026.

Where this paper and its answers come from

SSC does not publish CGL question papers or a public answer key. From 2025 the candidate response sheet prints every question and option as a picture, so the text here is transcribed from those pictures for this shift and checked against them independently; where the layout carries meaning, the picture is shown as well. Each answer is the key SSC colour-marked on that sheet. SSC released a tentative key for challenge and later its final one, both only to candidates behind a login, and the sheet does not say which of the two it carries. The solutions are written by UnlockIAS.

Argon is produced in Earth’s crust via?

(a)Decay of K-40
(b)Volcanoes
(c)Photosynthesis
(d)Cosmic rays

Answer

Why

Correct — A. Potassium-40, a naturally radioactive isotope in crustal rocks, decays with a half-life of about 1.25 billion years.

About 11% of those decays, by electron capture or positron emission, give stable argon-40. The rest, about 89%, give calcium-40.

That argon has built up over geological time, and nearly all of the atmosphere's argon is this radiogenic argon-40.

Why the others are wrong

(b)
Volcanoes — Volcanoes release gas that is already inside the Earth. The argon they vent was made earlier by potassium-40 decay; an eruption moves it, it does not produce it.
(c)
Photosynthesis — Photosynthesis releases oxygen and makes sugars. Argon is an inert noble gas, and no biological process produces it.
(d)
Cosmic rays — Cosmic rays do make a trace isotope, argon-39, but in the atmosphere and from argon-40. The argon produced in the crust comes from potassium-40.

Concept

Argon (Ar, atomic number 18) is a noble gas and the third most abundant gas in the atmosphere, at about 0.934% by volume, after nitrogen and oxygen.

That abundance is a radioactive legacy. Potassium is common in crustal minerals, a small fraction of it is potassium-40, and potassium-40 slowly decays to argon-40.

A rock traps the argon its own potassium produces, so the ratio of the two dates the rock: potassium–argon (K–Ar) dating.

Key facts

Argon makes up about 0.934% of the atmosphere by volume, third after nitrogen and oxygen.
Potassium-40 has a half-life of about 1.25 billion years, and about 11% of its decays give argon-40.
About 99.6% of atmospheric argon is argon-40.
K–Ar dating finds a rock's age from the ratio of argon-40 to potassium-40 in it.

Study next

Radiometric dating: carbon-14 versus potassium–argonUses of argon as an inert shielding gas in weldingIsobars, with argon-40 and calcium-40 as the textbook example

Common traps

Choosing "volcanoes" — they release stored gas, and the question asks how argon is produced.
Thinking cosmic rays make Earth's argon — they make trace argon-39 in the air, not the crust's argon-40.

Argon's uses are asked separately: 25 Sep 2024, 16:00, GA Q.10 keys argon as the gas used in welding titanium, aluminium, stainless steel and magnesium. Argon is also an option at 12 Sep 2024, 12:30, GA Q.21, on which gas glows red.

Related PYQs

No directly related past PYQ was found.

Topic: Scientific Research — all 152 SSC CGL questions on it, every solved shift.